With regard to a traditional duct for air-conditioning or ventilation, which has been conventionally employed for more than 100 years, one of the inventors of the present invention has proposed, a novel duct which can be assembled without generation of a noise, stored and transported with significantly increased efficiency, and can be suspended easily and at a free position, and a suspending member that suspends the duct from a ceiling or the like (Japanese Unexamined Patent Application Publication No. 10-47741, International Publication No. WO01/84058).
The novel duct and the suspending member attract attention from duct industries overseas including those of U.S. and Europe because of the above described features, as well as Japan.
One of the inventors made an innovative invention which can eliminate corner plates (corner members) which have been essential members in the prior art, by providing corner forming portions in corner portions of starting end and terminal end portions of the duct integrally with four plate members which are assembled by “seam joints” to form the duct having rectangular cross-section (Japanese Patent Application No. 2001-355906).
However, the corner forming portions are formed by another members and these members are integrally spot-welded to the plate members with these members positioned to protrude from end portions of the flange forming portions of the plate members. This construction reduces the number of assembly steps but significantly increases the number of parts and the number of producing steps.
Four plates (material in an expanded state before a forming process) forming the duct by joining seat joint portions at both ends are cut from a roll-shaped metal plate by a plasma cutter or the like. In a current manufacturing method, both end portions (corner portions) at a starting end portion and a terminal end portion of the plate are cut into scraps because they obstruct placement of the corner plates (corner members).
Various attempts have been made to manufacture “cornerplate-less duct” by improving the conventional duct manufacturing apparatus. However, in such a construction, if the portion to be unprocessed is positioned not to interfere with a roll portion (roll forming process region), then a roll diameter increases. As a result, the pitch (pitch at which the forming rolls are arranged) between forming rolls of adjacent forming roll pairs arranged along a flow of a forming process (forming process line or forming process step in substantially a direction in which the plate is traveled) increase.
If the dimension of the pitch of the forming rolls increases, it is difficult to process the plate with the plate positioned precisely with respect to the forming roll pairs which are going to perform the forming process. More specifically, since the plate is positioned at fewer locations and the pitch of arrangement of the forming rolls increases, the position of the plate which is to be processed may be displaced, or deformed. As a result, a plate member having a predetermined shape (dimension) cannot be manufactured.
Since the seam joint portions may become non-uniform, for example, it is difficult to manufacture the plate member having the predetermined shape (dimension) stably and with higher yield. The inventor has manufactured the plate member with trial and error, thereby resulting in only a yield of about 30 to 40%.
The present invention has been made under the circumstances. Attention has been focused on scraps which have been traditionally discarded. An object of the present invention is to provide a cornerplate-less duct having a rational construction, which enables the number of parts and forming steps to be significantly reduced by effectively utilizing the scraps, and enables a shape of an assembled duct to be maintained, a duct manufacturing method thereof, and a duct manufacturing apparatus (seam forming apparatus).